This invention relates to cool white fluorescent lamps and to two-component phosphors incorporated therein. More particularly it relates to cool white lamps that contain two-component phosphors and as a result yield higher lumens per watt with a suitable color rendition then do prior cool white lamps.
The color characteristics of light emitted from a fluorescent lamp depend on the choice of phosphors used to coat the internal walls of the lamp envelope. Emission spectra of luminescence centers in most phosphors consist of a single band peak at one particular wavelength. Therefore, in order to have white light it is necessary to either apply a mixture of phosphors or use a single phosphor containing more than one kind of luminescent center (such as the alkaline earth halophosphates). It is not enough to obtain the desired chromaticity coordinates and there are an infinite number of possible combinations of bands that would result in the same set of coordinates. It is also necessary that the lamp produce an acceptable luminous flux (brightness) and satisfactory optimum color rendition for all regions of the visible spectrum.
There are four standard lamps used today, daylight, cool white, white, and warm white and the desired chromaticity coordinates for these lamps are given hereinafter.
While it is possible to determine by theoretical computations the spectral energy distribution for a theoretical blue component and a theoretical yellow component that upon being blended together will yield a lamp having either brightness or color rendition maximized, such theory has to be tailored to the restraints as they exist in nature. In theory, a combination of a line emitting blue component and a line emitting yellow component would yield a lamp having the maximum brightness. Such a lamp however, cannot be produced for a number of reasons. First, phosphors having a line emission do not exist. Secondly, even if they existed the color rendition would be extremely poor because only two colors would be emitted and would result in color distortion in the area lighted by the lamp. Until recently the primary emphasis was placed upon color rendition with a suitable brightness. The single component halophosphates having two luminescent centers have been used to produce the aforementioned four white colors. The energy shortage, however, has shifted the emphasis to maximize lumens per watt of energy with an acceptable color rendition enabling a lower energy input to achieve the same level of brightness. While in theory, a two-component blend can produce warm white, there is no known binary combination of lumiphors that will yield that color, however, it has been discovered that binary blends can be made which will produce the other three colors.
U.S. Pat. No. 4,075,532 discloses that europium-activated barium magnesium aluminate can be used with a calcium fluoraapatite to achieve a cool white lamp. However, the teachings are primarily directed to the europium-activated strontium chloroapatite. In FIG. 3 of the foregoing patent the europium-activated strontium chloroapatite is represented by number 41 on the portion of the CIE diagram contained in that figure. The x and y coordinates for that phosphor are x=0.152 and y=0.027. No data are given for the europium-activated barium magnesium aluminate. The europium-activated barium magnesium aluminate has a higher y value than the europium-activated strontium chloropatite therefore when combined with a calcium fluorapatite the resulting combination will not yield a material having an emission within the cool white ellipse. Since the europium activated barium magnesium aluminate is a more efficient phosphor than europium activated strontium chloroapatite, it is desirable to utilize the aluminate material.
It is believed therefore that a two-component phosphor system which utilizes europium-activated barium magnesium aluminate and a halo phosphate phosphor to achieve a cool white lamp having a higher efficiency then the standard cool white lamp would be an advancement in the art.